I don't plan on maining a demon hunter but I enjoy the class as an alt. That being said, do you see the fall/decline of rogues, mages, and druids in high ranking pvp with demon hunters being able to see through stealth and invisibility? I honestly see them as the anti-stealth class tbh. (and if mages lose invis in Legion I apologize since I haven't tested my mage on beta yet)
Nah I don't think so. It's too early to tell anyway because they haven't even tuned numbers and nobody knows what the hell the best comps are going to be.
That's the problem with beta. Since nobody is seriously pushing rating, there's really no way to tell until the actual season starts.
Not at all. Spectral Sight has been changed over and over again and could easily be changed post-launch. Currently the Demon Hunter's movement speed is reduced while the spell is active and taking damage cancels the effect.
As of right now Rogue is a good counter to DH both solo and 3s set up. Rdrood always were/will be good in pvp with an exception of 1st Season in Wrath due to Dk's ability to dispel all hots.
Mages...well it is Blizzard's favorite child in pvp, they never will be bad at pvp.